228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



and radioiron (Fe 55, 59). Most of these radioisotopes emit either 

 beta radiation (high-speed electrons) or a mixture of beta and gamma 

 (electromagnetic radiation like X-rays). However, the energies of 

 radiation and the half-lives, that is, the rates vs^ith which the various 

 radioisotopes disintegrate, vary widely. 



The only radioisotope currently available under the distribution 

 program which emits alpha radiation (nuclei of helium atoms) is 

 polonium 210. Although this radioisotope exists in nature as one of 

 the decay products of radium and is commercially extracted from 

 radium wastes, it can now be obtained easier and cheaper by producing 

 it in the reactor by the irradiation of bismuth. 



HOW RADIOISOTOPES ARE USED 



Sources of radiation. — The first and simplest way of using radio- 

 isotopes is as sources of radiation. Most persons are familiar with 

 the way in w^hich radium and X-ray machines have been used to treat 

 certain diseases and to take pictures of heavy metal castings in looking 

 for possible cracks and flaws. Keactor-produced radioisotopes can 

 be used in much the same way. 



Tlie principal advantage of reactor-produced radioisotopes is that, 

 because there are a lot of them to choose from, the investigator has a 

 much wider choice of type and energy of radiation. Also, reactor- 

 produced radioisotopes are generally easier to handle and are much 

 cheaper. 



Tracers. — Radioisotopes or radioactive atoms are much more widely 

 used as tracer atoms — atoms that can be traced by the radiations they 

 emit. 



Since the radioactive atoms of an element are like the ordinary 

 nonradioactive or stable atoms of the element and behave like them 

 chemically, they go along with them in all chemical and biochemical 

 processes. But because of the radiations given off by the radioactive 

 atoms, they can act as "atomic detectives." With instruments such as 

 the geiger counter these radiations can be detected, that is, they can 

 be made to produce impulses or signals which may be seen or heard 

 or mechanically counted. This means that we can always locate the 

 radioactive atoms and hence distinguish between the atoms added to 

 a system and other atoms of the same element which were already 

 present. The use of radioisotopes in this way is referred to as the 

 tracer technique. 



POWER OF TRACER TECHNIQUE 



The tracer technique derives part of its power from its versatility. 

 We can label and trace almost any compound or material that we care 

 lo. Sometimes radiomaterials can be used in the simple chemical form 

 as shipped from Oak Ridge. This means as the element, as a simple 



